The present invention is directed generally to a power shift transmission and more specifically to a semi-automatic power shift transmission for use in such an industrial vehicle as forklift trucks of the type allowing an automatic shifting operation to be performed at a controlled shift point of time, regardless of the driver""s untimely shifting command which may cause an overheat and damage of the transmission. The invention also discloses a shift control method that assures a well-timed, smooth shifting operation of a semi-automatic power shift transmission.
As ordinary artisan are aware, industrial vehicles such as a forklift truck have been employed for the load shipping purpose or for the transportation of heavyweight articles from one place to another, usually within a limited working area, The industrial vehicles normally employ a power shift transmission to perform gear shifting between a forward gear, a reverse gear and a neutral gear by use of fluid pressure. The transmission can be up-shifted or down-shifted in the forward gear between first and second gear ratios.
For the explanation of conventional power shift transmissions typically incorporated in the forklift truck, certain major components associated with the gear shifting operation are illustrated in FIG. 1 by way of example.
As illustrated, the forklift truck includes a starter switch 10, an engine 12, a shift lever 14, a transmission control unit 16, a torque converter 17 and a transmission main body 18 equipped with three solenoid valves 20, 22, 24. The starter switch 10 can be manually actuated to energize a start motor(not shown) which in turn provides initial driving force to the engine 12. The shift lever 14 is manipulated by the driver to select a proper gear ratio of the truck. The information on the gear ratio selected by the driver is given to the transmission control unit 16.
The transmission control unit 16 is adapted to generate and feed control signals to the three solenoid valves 20, 22, 24 on the basis of the information obtained from the shift lever and other sensors. The solenoid valves 20, 22, 24 are selectively energized or de-energized, depending upon the control signals from the transmission control unit 16. Combination of energization and de-energization of the solenoid valves 20, 22, 24 establishes a particular hydraulic circuit, thus changing the gear ratio of a planetary gear device within the transmission main body 18.
It is widely known in the art that the torque converter 17 serves to provide torque multiplication and hydrodynamic damping benefits in transmitting power from the engine 12 to the transmission main body 18. In the meantime, the stop lover 26 can be optionally operated during the course of shifting operation either to temporarily stop the engine 12 or reduce the engine output power by virtue of restricting fuel supply to the engine 12. This helps avoid shift shock which would otherwise take place in the shifting operation.
The typical power shift transmission of the construction as set forth above, however, has a shortcoming in that the transmission tends to be overheated and eventually damaged as it is frequently exposed to untimely shifting operation. Specifically, there are many instances where the forklift truck driver tries to change the gear ratio in an untimely manner at an improper travelling speed of the forklift truck, which may lead to an increased shift shock and an exceedingly high heat generation in the torque converter.
In case of the forklift truck among other industrial vehicles, it has come to the inventor""s attention that the angle of a steering wheel should be taken into account to inhibit any up-shift at a great steering angle which would otherwise result in an excessive heat generation in various parts of the transmission and, sometimes, traffic accidents due to the quickly increased truck speed at a heavy steering angle.
Accordingly, it is an object of the invention to provide a semi-automatic power shift transmission for use in an industrial vehicle and a shift control method thereof capable of automatically changing or shifting the gear ratio at a well-controlled shift time and with a reduced shift shock.
Another object of the invention is to provide a semi-automatic power shift transmission for use in an industrial vehicle and a shift control method thereof which can take into consideration the steering wheel rotation angle to avoid any up-shift of the gear ratio at a great steering angle.
With these objects in view, one aspect of the present invention lies in the provision of a semi-automatic power shift transmission for use in an industrial vehicle, comprising: a manual shift lever with at least first and second gear positions; a hydraulically shifted transmission gear mechanism operable in one of at least first and second gear ratios corresponding to the first and second gear positions; electromagnetic actuator means for, when energized, causing the transmission gear mechanism to be either up-shifted from the first to the second gear ratio or down-shifted from the second to the first gear ratio; a transmission speed sensor for detecting the output speed of the transmission gear mechanism to generate a transmission speed signal; an engine speed sensor for picking up the speed of a vehicle engine to produce an engine speed signal; a steering wheel angle sensor for measuring the steering wheel angle to issue a wheel angle signal; and a transmission control unit responsive to the transmission speed signal, the engine speed signal and the wheel angle signal for selectively energizing the electromagnetic actuator means to thereby carry put the up-shifting operation in the event that a load ratio denoting the transmission speed divided by the engine speed is greater than a preselected reference ratio, with the transmission speed larger than a preselected reference up-shift speed and the steering wheel angle smaller than a preselected reference angle.
Another aspect of the present invention resides in the provision of a shift control method of a semi-automatic power shift transmission for use in an industrial vehicle, comprising the steps of: a) detecting the output speed of a transmission gear mechanism to generate a transmission speed signal; b) picking up the speed of a vehicle engine to produce an engine speed signal; c) calculating a load ratio by way of dividing the transmission speed with the engine speed; d) determining whether the calculated load ratio is greater than a preselected reference load ratio; e) if the answer at step d) is affirmative, determining whether the transmission speed is greater than a preselected reference up-shift speed; f) if the answer at step e) is affirmative, measuring the angle of a steering wheel and determining whether the steering wheel angle is loss than a preselected reference angle; and g) if the answer at step f) is affirmative, executing the up-shift operation of the transmission gear mechanism from a first gear ratio to a second gear ratio.